The 45/70 is a good round, and responds favorably to being stoked alot hotter. I have a 1895, the longer sibling of the guide gun, and would have no problem going after elk with it. Make sure you load decent bullets however. Hard cast or better jacketed rounds over the cheap remington bullets IMO. The cheap ones tend to not like the hotter loadings. I would also throw the 444, and 450 marlin in that box as well.
As for higher velocity loads, the venerable '06, 7mm remington mag, 300 win mag, etc are all capable, just use heavy for caliber bullets. Even the 375H&H is great for elk, shooting as flat as an '06, and hits like a hammer. Slower guns like the big boomer lever guns are great shorter range guns. The magnums are better at intermediate to longer range shots.
The real question is, what kind of hunter are you, as in, the kind that likes or takes longer shots, or prefers closer up hits? That will really dictate what to get.